Control of industrial spray dryers has traditionally attempted to regulate the moisture content of the dried product. Marketing pressures increasingly require closer control of both wetness and of other powder properties, including bulk density, solubility and wettability. We argue that, with the exception of changes to the gross heat load of the dryer, the major influence on the dried powder is the particle size distribution of the spray cloud. Consequently, the internal condition of the spray in the dryer must be controlled to satisfactorily maintain the state of the dried product. A Malvern Instruments Particle Sizer has been installed in a pilot-plant, tall-form spray dryer with a two-fluid nozzle to infer the spray droplet volume-surface area mean diameter. A program was implemented in FORTRAN on a VAX 11/730 computer to control the volume-surface area mean diameter via cascaded control of the nozzle-air flowrate. Experiments spraying 2 wgt% sodium nitrate solution have shown that the volume-surface area mean diameter of particles in the droplet cloud can be controlled by manipulation of the atomizing air flowrate.